Phenolic type resins are commonly used to impregnate Kraft paper core sheets used in the manufacture of decorative laminates, as taught by Meier, in U.S. Pat. No. 3,897,589, where the core is bonded to a decorative sheet of high quality alpha cellulose paper impregnated with a melamine resin. Meier taught the use of a variety of phenol component materials for Kraft paper impregnation, including phenol itself, cresol (methyl phenol), xylenol (dimethyl phenol), resorcinol (benzene diol), and naphthol (hydroxy napthalene).
O'Donnell, in U.S. Pat. No. 3,216,884, teaches the use of other phenolic type resins, as co-impregnants with polyester resins for cellulosic core sheets of printed circuit boards which are required to have good moisture resistance, good punching ability, and good electrical properties. The impregnating emulsion comprises droplets of phenolic resin dissolved in a diluent and suspended in a polyester resin. O'Donnell teaches a wide variety of phenol component materials including phenol itself or substituted phenols where hydrogen is substituted for by: F, Cl or Br; alkyl groups having from 1 to 18 carbons; alicyclic groups having from 5 to 18 carbons; aromatic or aralkyl groups having from 6 to 18 carbons; alkyl, alicyclic, aryl and aralkyl ketones; and the like.
Despite the widespread use of phenolic and melamine resins in the preparation of decorative sheets, thin laminates of large areas prepared from such resin combinations have a tendency to warp when subjected to substantial variations in humidity and temperature conditions. Warpage in decorative laminates can adversely affect the trimming, fabrication and eventual stability of the article prepared from the laminates, such as a wall panel or counter top. A number of theories exist which attempt to explain the cause of warpage in decorative laminates, among which are: the difference in cure rate of phenolic and melamine resin impregnated sheets; the difference in moisture absorption of melamine and phenolic resin in the cured state; and the differential rate of expansion of the various Kraft and alpha cellulose sheets which constitute the decorative laminate.
Another problem encountered in the manufacture of decorative laminates is the occurrence of dry spots, i.e., non-uniformity in appearance and composition of the core portion of the laminate. This non-uniformity in appearance and composition can normally be attributed to a lack of adequate penetration of the phenolic resin during the impregnation process, or inadequate resin flow in the "B"-staged impregnated sheet. Laminates prepared from non-uniform sheets normally have poor bond strength and are deficient in other mechanical and physical properties. This problem has become more acute recently because there is a trend toward using higher basis weight Kraft paper core sheets and higher solids resins in the decorative laminating industry. There has been a longfelt need then, for a new, more easily saturable core paper having a rate of expansion closer to alpha cellulose paper, or either a new phenolic or melamine resin system which would have good flow and penetration characteristics and better compatibility with the other resin used in the laminate.